In an optical Local Area Network (hereinafter referred to as LAN), light signals are transmitted and received at a transmission speed of several Gb/s (Gigabit per second). However, it is expected that along with increasing amounts of communications information, light signals would have to be transmitted and received at a transmission speed of 10 or more Gb/s in the future. A light detection device used in the present optical LAN mainly employs a PIN photo diode (hereinafter referred to as PIN-PD), and the PIN-PD can respond to a transmission speed of several Gb/s, but cannot sufficiently respond to still higher transmission speeds. Therefore, a photodetector which can operate at a speed still higher than that of the PIN-PD is desired.